1. Field of the Invention
The invention relates to the calibration of an articulating probe head for a coordinate positioning machine. Coordinate positioning machines, such as coordinate measuring machines or machine tools, comprise an arm and a table movable relative to each other, typically with three linear degrees of freedom. Such machines may be equipped with a probe to enable inspection of the dimensions or contour of components. It is frequently desirable to orient the probe relative to the arm of the machine upon which it is mounted, in order to inspect differently oriented surfaces of a component.
2. Description of Related Art
One known-device providing two-axis orientation of a probe relative to the arm of the machine is the Renishaw PH9 probe head, which orients a probe into one of 720 discrete repeatable positions provided substantially about first and second orthogonal axes. The position of the probe in each of the 720 positions (or as many of the positions as are required) is pre-calibrated; because each of the positions is highly repeatable, i.e. each time the probe is oriented to a given position, it occupies substantially the same position in space, the calibration of the probe position takes account of errors such as droop of the probe under the influence of gravity.
A further type of probe head is known from U.S. Pat. No. 4,888,877, and provides continuous two-axis orientation of a probe relative to the movable arm, with the orientation of the probe being measured by encoders within the probe head.
Errors such as axis eccentricity, and non-perpendicularity of axes are pre-calibrated; errors due to droop are corrected by calculating the droop of the probe for a given angular orientation, and introducing a corresponding correction value into measurements subsequently made with the probe head at the said orientation (as shown in U.S. Pat. No. 5,138,563).